Exhaust gas purifying catalyst

ABSTRACT

In an exhaust gas purifying catalyst, an acid material with a high affinity with respect to an absorbing agent is dispersed and mixed in a catalyst layer, to which the absorbing agent is added, or a layer of the acid material is formed inside the catalyst layer in order to prevent the absorbing agent from moving from the catalyst layer into the carrier. This reduces the permeation of the absorbing agent added to the catalyst layer into a carrier, the evaporation and splash of the absorbing agent from the catalyst, and the deterioration in the durability and the exhaust gas purifying performance of the catalyst.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an exhaust gas purifyingcatalyst, and more particularly to an exhaust gas purifying catalystthat is able to keep a high purifying performance.

2. Description of Related Art

A lean combustion type engine such as a lean burn engine and a cylinderfuel injection type engine is run at a lean air-fuel ratio that is leanthan a stoichiometrical air-fuel ratio, in a predetermined running rangein order to improve a fuel consumption characteristic and an exhaust gascharacteristic. While the engine is run at the lean air-fuel ratio, athree way catalyst cannot satisfactorily purify NOx (nitrogen oxide) inan exhaust gas. Therefore, it is well known that there is provided anNOx catalyst that absorbs the NOx in the exhaust gas in an oxideatmosphere, and the NOx absorbed by this catalyst is reduced to N₂(nitrogen) in an reducing atmosphere to thereby reduce the amount of NOxdischarged into the air. An example of such an occlusion-type lean NOxcatalyst is disclosed in Japanese Patent Provisional Publication No.9-85093, potassium (K) as one of alkali metals is added as an NOxabsorbing agent to the catalyst in order to improve an NOx absorbingperformance.

If, however, the NOx catalyst to which the potassium is added is used ata high temperature for a long period of time, the catalyst may becracked. This results in the deterioration in the durability of the NOxcatalyst.

In order to discover the cause of the deterioration in the durability,the inventors of the present invention manufactured an NOx catalyst, inwhich potassium as one of alkali metals is added as an NOx absorbingagent to a catalyst layer held in a honeycomb cordierite carrier (aporous carrier), and conducted a bench test of an engine equipped withthis NOx absorbing agent and a running test of a vehicle provided withthis engine. In the bench test and the vehicle running test, the engineand the vehicle were run under the condition that the NOx catalyst wasexposed to a high temperature of not less than 650° C. for a long periodof time. After the running of the engine and the vehicle, an elementanalysis was conducted with respect to a cut surface of the NOx catalystby an EPMA method (an electron beam probe micro part analysis method).As a result, it was found that a compound of KMg₄Al₉Si₉O₃₆ of potassium,magnesium, aluminum, silicon and oxygen and a compound KAlSiO₄ ofpotassium, aluminum, silicon and oxygen were present in a cordierite(Mg₂Al₄Si₅O₈) layer of the catalyst.

According to the above tests, if the NOx catalyst is exposed to a hightemperature, the potassium added to the catalyst layer (a wash coat)permeates the cordierite carrier, and reacts with the cordierite in ahightemperature atmosphere. It can be considered that the potassiumeasily permeates the cordierite carrier because the potassium compoundhas a high water solubility and a low fusing point. When a compound witha different coefficient of thermal expansion from the cordierite isformed in the cordierite carrier, the cordierite carrier is cracked withthe change in a catalyst temperature during the use of the catalyst andbefore and after the use of the catalyst.

As stated above, the NOx catalyst including the potassium and the likeas the absorbing agent is used in the oxide atmosphere.

In the oxide atmosphere, the absorbing agent chemically reacts withnitrogen components and sulfur components in the exhaust gas to therebyform a nitrate and a sulfate of the catalyst. This deteriorates the NOxabsorbing performance. The absorbing performance can be recovered byforming the reducing atmosphere around the NOx catalyst and dissolvingthe nitrate and the sulfate. In this case, however, the purifyingperformance may be deteriorated if the NOx catalyst is used at a hightemperature for a long period of time.

According to the results of the tests conducted by the inventors of thepresent invention, one of the causes of the deterioration in thepurifying performance is considered to be that the absorbing agent isgradually evaporated and splashed from the NOx catalyst at a hightemperature and therefore a considerable amount of the absorbing agentis dissipated. More specifically, the inventors of the present inventionmanufactured the NOx catalyst in which the catalyst layer including thepotassium as the absorbing agent is held in the cordierite carrier, andfound a potassium content of an unused NOx catalyst by an XRF method (anX-ray fluorescence analysis method). Then, they found the potassiumcontent of the catalyst after the use of the catalyst at a hightemperature for a long period of time (e.g., at 850° C. for 32 hours),and then found the dissipated amount of the potassium by dividing adifference in the potassium content before and after the use of thecatalyst by the original potassium content. Consequently, the dissipatedamount of the potassium was found to be dozens of % to 50%.

It is therefore an object of the present invention to provide an exhaustgas purifying catalyst that is able to significantly reduce the degreeto which the exhaust gas purifying performance is deteriorated due tothe dissipation of the absorbing agent.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an exhaustgas purifying catalyst that is able to significantly reduce the degreeto which the exhaust gas purifying performance is deteriorated due tothe dissipation of the absorbing agent.

The above object can be accomplished by providing an exhaust gaspurifying catalyst, which includes a carrier and a catalyst layer and inwhich at least one of alkali metals and alkali earth metals is added asan absorbing agent to the catalyst layer, the exhaust gas purifyingcatalyst wherein: an inhibiting agent is provided in the catalyst layerin order to inhibit the movement of the absorbing agent in the catalyst.This inhibits the movement of the absorbing agent in the catalyst, andprevents the dissipation of the absorbing agent due to the evaporation,splash, etc. of the absorbing agent from the catalyst, and thedeterioration in the exhaust gas purifying performance of the catalyst.

Preferably, the inhibiting agent is dispersed and mixed in the catalystlayer, or is provided in a form of a layer in the catalyst. Onlydiffusing and mixing the inhibiting agent in the catalyst layer inhibitsthe movement of the catalyst, but the movement of the inhibiting agentcan be surely prevented by providing the inhibiting agent in the form ofthe layer.

Preferably, the inhibiting agent includes an acid oxide including atleast one acid substance selected from transition elements of IV, V andVI groups and typical elements of IV, V and VI groups; a composite oxideincluding the at least one acid substance; and at least one materialselected from a group composed of a material that never disturbs areactivity between a nitrogen oxide and the absorbing agent and amaterial that absorbs a reduced substance. In this case, the inhibitingagent may include zeolite or include an acid oxide comprised of at leastone acid substance among silica, titanium and tungsten.

Preferably, the absorbing agent includes potassium, and the carrier iscomprised of a porous carrier.

Moreover, the layer of the inhibiting agent preferably comprises atleast one of the following: a layer with a high acidity, a layer with alarge specific surface, a layer with a small crystal lattice, a layercomposed of an element compound with a heavy molecular weight, and alayer with a high basicity.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature of this invention, as well as other objects and advantagesthereof, will be explained in the following with reference to theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures and wherein:

FIG. 1 is a partially-enlarged cross-sectional view showing a quarter ofa shell in an exhaust gas purifying catalyst according to a firstembodiment of the present invention;

FIG. 2 is a view showing the affinity with respect to potassium of anacid material;

FIG. 3 is a view showing a potassium content of a catalyst layer afteran exhaust gas purifying catalyst is used at a high temperature for along period of time;

FIG. 4 is a view showing an NOx purifying efficiency of an exhaust gaspurifying catalyst after the exhaust gas purifying catalyst is used at ahigh temperature for a long period of time;

FIG. 5 is a partially-enlarged cross-sectional view showing a quarter ofa shell in an exhaust gas purifying catalyst according to a secondembodiment of the present invention in the case where an acid materialis composed of particles or blocks;

FIG. 6 is a conceptual view showing a potassium fixing operation by acation exchange ability of zeolite;

FIG. 7 is a partially-enlarged cross-sectional view showing a quarter ofa shell in an exhaust gas purifying catalyst according to a thirdembodiment of the present invention;

FIG. 8 is a conceptual view for describing the state wherein a silicalayer is formed inside a pore of a cordierite carrier;

FIG. 9 is a partially-enlarged cross-sectional view showing a quarter ofa shell in an exhaust gas purifying catalyst according to a fourthembodiment of the present invention;

FIG. 10 is a partially-enlarged cross-sectional view showing a quarterof a shell in an exhaust gas purifying catalyst according to a fifthembodiment of the present invention;

FIG. 11 is a conceptual view showing a potassium fixing operation by acation exchange ability of zeolite, which constitutes an inhibitionlayer of a catalyst in FIG. 10;

FIG. 12 is a view showing a potassium content after a catalyst in FIG.10 is used at a high temperature for a long period of time, comparedwith an original catalyst, a catalyst in FIG. 7 and a catalyst in FIG.9; and

FIG. 13 is a view showing an NOx purifying efficiency after a catalystin FIG. 10 is used at a high temperature for a long period of time,compared with an original catalyst, a catalyst in FIG. 7 and a catalystin FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of an exhaust gas purifying catalyst according tothe present invention will be described hereinbelow.

First, a description will be given of the first and second embodimentswherein an inhibitor is dispersed and mixed in a catalyst layer.

The exhaust gas purifying catalyst is an NOx catalyst having anhoneycomb cordierite carrier composed of many cells. FIG. 1 shows a partof a cell in the cordierite carrier. The cell of the cordierite carrier10 is, for example, quadrangular. A catalyst layer 20 is held on asurface of the cordierite carrier 10. The cordierite carrier 10 isproduced by, for example, mixing alumina powder, silica powder andmagnesia powder so that the ratio thereof can conform to the compositionof a cordierite, diffusing the mixed powder in water, forming a solidcontent thereof in the shape of a honeycomb, and sintering the honeycombcompact.

For example, the catalyst 20 is formed in a manner described below.First, a slurry including a noble metal such as platinum, an alkalimetal or an alkali earth metal such as potassium (K) and barium (Ba) asan NOx absorbing agent, an acid material (an inhibitor) 30 such assilicon (Si), and the like is prepared. Then, the cordierite carrier 10is immersed in the slurry, and is dried and sintered. Consequently, theacid material 30 is mixed in the catalyst layer including the noblemetal and the alkali metal or the alkali earth metal.

The NOx absorbing agent is typically formed of potassium (K) or barium(Ba), but the present invention is not restricted to this specificexample. The NOx absorbing agent may be formed of any kinds of alkalimetals or alkali earth metals. The acid material (the inhibitor) 30 istypically formed of silica (silicon oxide), but as shown in FIG. 2, theoxide material 30 may be formed of any kinds of transition elements ofIV, V and VI groups or typical elements (e.g., P, S, V, Cr, As, Nb, Mo,W) of IV, V and VI groups. Preferably, the acid material 30 has a highaffinity with respect to the alkali metal or the alkali earth metal asshown in FIG. 2 (FIG. 2 shows the affinity with respect to the potassiumfor example). If a reactivity with respect to the NOx absorbing agent istaken into consideration, the acid material 30 is preferably silicon(Si) or tungsten (W) in the case where the NOx absorbing agent is formedof the potassium. Preferably, the acid material 30 never disturbs thereactivity with respect to the NOx and the NOx absorbing agent.

The acid material 30 may be a composite material if it has the affinitywith respect to the NOx absorbing agent. Therefore, the acid material 30may be zeolite, which has a cation exchange ability equivalent to theaffinity.

The above processing acquires an NOx catalyst in that the cordieritecarrier 10 is coated with the catalyst layer 20. As is well known, theNOx catalyst is contained in a case through a cushioning material, andthe case is arranged in an exhaust pipe of a lean bum internalcombustion engine.

The NOx catalyst absorbs NOx as nitrate in an exhaust gas under theoperation of catalyst species dispersed in the catalyst layer 20 whilethe engine is run at a lean air-fuel ratio. When the engine is run at arich air-fuel ratio, the nitrate is dissolved, and the absorbed NOx isreduced to nitrogen and is emitted from the NOx catalyst into the air.

If the internal combustion engine provided with the NOx catalyst is runfor a long period of time, the NOx catalyst is exposed to a hightemperature. If the NOx catalyst is a conventional NOx catalyst whosecordierite carrier is coated with a catalyst layer to which thepotassium or the barium (hereinafter only referred to as the potassium)as the NOx catalyst is only added, the potassium moves to the cordieritecarrier to react with silica components and the like in the carrier toproduce a compound and crack the cordierite carrier. This damages thedurability of the NOx catalyst.

According to an elementary analysis by an EPMA method, the NOx catalystof the present embodiment prevents the production of the compound of thepotassium added to the catalyst layer 20 and the silica components ofthe cordierite carrier 10 even if the NOx catalyst is used at a hightemperature for a long period of time. This is because the acid material30 such as the silicon as well as the potassium is mixed in the catalystlayer 20, and therefore the potassium is dispersed in and is attractedto acid material particles due to the affinity of the acid material 30so that the potassium can desirably be held without moving in thecatalyst layer 20.

When the potassium content of the catalyst layer 20 was measured afterthe NOx catalyst was used at a high temperature for a long period oftime, it was found that a considerable amount of potassium stillremained in the catalyst layer 20 compared with the case where thecatalyst layer, in which the acid material was not mixed, was used as inthe prior art (indicated by a broken line).

Moreover, the reason why a considerable amount of potassium remains inthe catalyst layer 20 is as follows. In the case of the conventional NOxcatalyst, the nitrate of the potassium has a low fusing point, and thus,the potassium easily moves in the catalyst if it is exposed to a hightemperature, and is easily evaporated and splashed due to a low boilingpoint of the potassium. On the other hand, the NOx catalyst of thepresent embodiment reduces the evaporation and splash of the potassiumdue to the affinity with respect to the acid material 30, and therefore,the potassium is steadily held in the catalyst layer 20.

Particularly if the catalyst layer 20 includes zeolite as the acidmaterial 30, a more satisfactory result can be achieved since thezeolite has the cation exchange ability.

More specifically, the NOx absorbing agent such as the potassium movingin the NOx catalyst may be ionized under the presence of hightemperature vapor. If the catalyst layer 20 includes the zeolite, theNOx absorbing agent is fixed as ions due to the cation exchange abilityof an acid point on the zeolite as shown in FIG. 6, and this preventsthe movement of the NOx absorbing agent toward the carrier.

The zeolite has a three-dimensional mesh structure, and has a largespecific surface. Thus, the zeolite is capable of screening molecules.The NOx absorbing agent such as the potassium is highly dispersed on thezeolite, and thus, further inhibits the movement of the NOx absorbingagent into the carrier.

Even when the internal combustion engine is run at the lean air-fuelratio, the exhaust gas includes a small amount of HC. The zeolite has anexcellent performance to fix the NOx absorbing agent and absorb the HC,and the HC absorbed onto the zeolite promotes the dissolution of thenitrate and sulfate of the NOx absorbing agent. More specifically, whenthe internal combustion engine is run at the lean air-fuel ratio, thezeolite having the HC absorbing performance continuously dissolves thenitrate and sulfate of the NOx absorbing agent by using the small amountof HC included in the exhaust gas, thereby recovering the NOx absorbingperformance of the catalyst.

The zeolite layer 150 may be formed of various types of zeolite such assuch as MFI type, Y type, X type, mordenite and ferrielite. It isnecessary to select zeolite which conforms to the composition of theexhaust gas in view of the structure relevancy with an absorption HCspecie.

The cation exchange ability and the heat-resisting performance of thezeolite depend on the composition of the zeolite. More specifically, thecation exchange ability is in inverse proportion to a ratio SiO₂/AlO₂ ofthe zeolite, and the heat-resisting performance is in proportion to thisratio. Therefore, increasing this ratio as much as possible improves theheatresisting performance of the catalyst, and decreasing the ratioreduces the dissipated amount of the NOx absorbing agent when thecatalyst is used at a high temperature for a long period of time.

As stated above, the exhaust gas purifying catalyst of the presentinvention can steadily hold the NOx absorbing agent such as potassium inthe catalyst layer 20 without moving and splashing it. This prevents theproduction of a compound with a different coefficient of thermalexpansion from that of the cordierite carrier 10 in the cordieritecarrier 10 to thereby prevent the crack of the cordierite carrier 10,which results from the production of the compound, and improve thedurability of the exhaust gas purifying catalyst. This maintains theexcellent exhaust gas purifying performance.

When the NOx purifying efficiency of the NOx catalyst was checked afterthe NOx was used at a high temperature for a long period of time, it wasfound that the present invention maintained the higher NOx purifyingefficiency regardless of the catalyst temperature as indicated by asolid line in FIG. 4 compared with the case where the catalyst layer, inwhich the acid material was not mixed, was used as in the prior art(indicated by a broken line).

One of substances for deteriorating the purifying performance of the NOxcatalyst is a sulfate comprised of sulfur components. The exhaust gaspurifying catalyst of the present invention can disperse and hold theNOx absorbing agent such as potassium in the catalyst layer 20, and thisprevents the growth of such a sulfate.

In the above-described first embodiment, the acid material 30 is mixedas fine powder in the catalyst layer 20, but the acid material 30 may berelatively large particles or blocks if it is mixed in the catalystlayer 20. The present invention can be applied favorably to such a case(the second embodiment).

In the above-described first embodiment, the honeycomb cordieritecarrier is used as a porous carrier, but the present invention can alsobe applied to an exhaust gas purifying catalyst provided with a carrierformed of material other than the cordierite. The use of a metal carrierprevents the splash of the absorbing agent and prevents thedeterioration of the exhaust gas purifying performance of the catalyst,although the prevention of the permeation of the NOx absorbing agentinto the carrier is nearly out of question. The honeycomb cordieritecarrier is used, the cells thereof are not only quadrangular but alsotriangular and hexagonal.

There will now be described the first through fifth embodiments of thestructure in which a layer of the acid material is formed in thecatalyst layer.

As is the case with the first embodiment, an exhaust gas purifyingcatalyst of the third through fifth embodiments has a honeycomb(monolith) cordierite carrier composed of many cells. FIG. 7 shows apart of a cell in the cordierite carrier. For example, the cells of acordierite carrier 110 are quadrangular. A surface of the cordieritecarrier 110 is coated with a silica layer 120, and a catalyst layer 130is held on a surface of the silica layer 120. Potassium (K) and barium(Ba) are added as an NOx absorbing agent to the catalyst layer. Thesilica layer 120 functions as an inhibition layer for inhibiting thepermeation of the potassium into the cordierite carrier 110 (moregenerally, a porous carrier).

The cordierite carrier 110 is produced by, for example, mixing aluminapowder, silica powder and magnesia powder so that the ratio thereof canconform to the composition of a cordierite; diffusing the mixed powderin water; forming a solid content thereof in the shape of a honeycomb;and sintering the honeycomb compact.

For example, the silica layer 120 is formed on the surface of thecordierite carrier 110 in a manner described below. First, water-solublesalt of a silicon compound is diluted by water to prepare an aqueoussolution with a predetermined concentration, and the cordierite carrier110 is immersed in the aqueous solution. The aqueous solution of salt ofthe silicon compound is absorbed into the surface and a surface layer ofthe cordierite carrier 110 due to the hydrophilia of the cordierite 110.Then, the cordierite carrier 110 is dried to evaporate the watercontent, and the salt of the silicon compound is absorbed into thesurface and the surface layer of the cordierite carrier 110. When thecordierite carrier 110 is heated, the salt of the silicon compound isdissolved to form the silica layer 120 on the surface of the cordieritecarrier 110. In short, the cordierite carrier is coated with the silicalayer 120.

An optimum concentration of the aqueous solution of salt of the siliconcompound used for the formation of the silica layer 120 is mainlychanged according to the hydrophilic characteristic of the cordieritecarrier

Accordingly, the elementary analysis is preferably performed withrespect to the surface layer of the cordierite carrier by the EPMAmethod or the like in order to confirm a relationship between theconcentration of the aqueous solution and the coated state in advance.Finding the optimum concentration of the aqueous solution of the salt ofthe silicon compound in advance acquires the optimum coated state thatensures the adhesiveness between the cordierite carrier and the catalystlayer, and prevents the permeation of the potassium into the silicalayer. For example, the catalyst layer 130 is formed on the surface ofthe silica layer 120 in a manner described below. First, a slurryincluding powder that is mainly composed of noble metal such asplatinum, an alkali metal such as the potassium, and an alkali earthmetal such as barium is prepared. Then, the cordierite carrier 110coated with the silica layer 120 is immersed in the slurry and is driedand sintered.

This obtains an NOx catalyst in which the cordierite carrier 110 iscoated with the catalyst layer 130 through the silica layer 120. As iswell known, this NOx catalyst is contained in a case through acushioning member, and is arranged in an exhaust pipe of a leancombustion internal combustion engine.

This NOx catalyst absorbs NOx as nitrate as in exhaust gases under theoperation of catalyst species dispersed in the catalyst layer 130 whilethe engine is run at a lean air-fuel ratio. While the engine is run at arich air-fuel ratio, the NOx catalyst dissolves the nitrate and reducesthe absorbed NOx to nitrogen, which is emitted from the NOx catalystinto the air.

If the internal combustion engine provided with the NOx catalyst is runfor a long period of time, the NOx catalyst is exposed to a hightemperature. If the NOx catalyst is a conventional NOx catalyst whosecordierite carrier is coated with a catalyst layer to which thepotassium is added, the potassium moves to the cordierite carrier toreact with silicon and the like in the carrier to produce a compound andcrack the cordierite carrier as described previously. This damages thedurability of the NOx catalyst. According to the elementary analysis bythe EPMA method, the NOx catalyst of the present embodiment prevents theproduction of the compound of the potassium added to the catalyst layer20 and the silica components of the cordierite carrier 10 even if theNOx catalyst is used at a high temperature for a long period of time.This is because the silica layer 120 prevents the potassium from movingfrom the catalyst layer 130 into the cordierite carrier 110. Since acompound with a different coefficient of thermal expansion from thecordierite carrier 110 is not produced in the cordierite carrier 110,the cordierite carrier 110 can be prevented from being cracked due tothe production of such a compound.

There will now be described an exhaust gas purifying catalyst accordingto the fourth embodiment of the present invention.

As shown in FIG. 9, the exhaust gas purifying catalyst of the fourthembodiment is different from that of the third embodiment in that atitania layer 140, which is comprised mainly of a titanium dioxide(TiO₂), is formed as an inhibition layer instead of the silica layer120. Otherwise, the exhaust gas purifying catalyst of the presentembodiment has the same structure as that of the third embodiment. Theexhaust gas purifying catalyst of the present embodiment can bemanufactured in substantially the same manner as that of the thirdembodiment.

According to the elementary analysis by the EPMA method, the exhaust gaspurifying catalyst of the present embodiment in which the titania layeris formed between the cordierite carrier 110 and the catalyst layer 130,the potassium added to the catalyst layer 130 is prevented frompermeating the cordierite carrier 110 even if the catalyst is used at ahigh temperature for a long period of time. Since the permeation of thepotassium is prevented, the exhaust gas purifying catalyst of thepresent embodiment has an excellent durability, and reduces the loss ofthe potassium from the catalyst layer 130 for the same reason as is thecase with the silica layer.

There will now be described an exhaust gas purifying catalyst accordingto the fifth embodiment of the present invention.

As shown in FIG. 10, the exhaust gas purifying catalyst of the fifthembodiment is different from that of the third embodiment in that azeolite layer 150 is formed as an inhibition layer, instead of thesilica layer 120. Otherwise, the exhaust gas purifying catalyst of thepresent embodiment has the same structure as that of the thirdembodiment. The exhaust gas purifying catalyst of the present embodimentcan be manufactured in substantially the same manner as that of thethird embodiment.

When the zeolite layer 150 is formed in the cordierite carrier 110,zeolite components may be dispersed in an aqueous dispersing agent as isthe case with the third embodiment, but the zeolite components may bedispersed may also be dispersed in an organic dispersing agent. It ispossible to use an underwater dispersed matter (sol) and an electrifieddiffusing solution (colloid) of a hydrate of silica, alumina or thelike.

In the catalyst of the present embodiment provided with the zeolitelayer 150 as the inhibition layer, the zeolite layer 150 has an acidpoint with a cation exchange ability, and has an excellent ability tofix an absorbing agent (potassium in the present embodiment). Theabsorbing agent moving in the catalyst may be ionized under the presenceof vapor at a high temperature. As shown in the conceptual drawing ofFIG. 11, the absorbing agent such as the potassium is fixed as ions dueto the cation exchange ability of the acid point on the zeolite layer150. The zeolite layer 150 has a large specific surface due to itsthree-dimensional mesh structure. The potassium is highly dispersed onthe zeolite that is constructed in the above-mentioned manner, and thismakes it difficult for the potassium to permeate the cordierite carrier110. The zeolite layer 150 has an excellent ability to absorb a reducedsubstance (e.g., reduced gas such as HC). Even when the internalcombustion engine is run at the lean air-fuel ratio, the exhaust gasincludes a slight amount of HC, and the HC absorbed onto the zeolitelayer 150, which is capable of absorbing HC, facilitates the dissolutionof the nitrate and the sulfate of the potassium. More specifically, thezeolite layer 150 continuously dissolves the nitrate and the sulfate byusing the slight amount of HC included in the exhaust gas in order torecover the NOx absorbing performance of the catalyst.

The zeolite layer 150 of the present embodiment does not include anycatalyst substances such as noble metal (e.g., platinum), and therefore,the platinum and the like do not take a catalytic action in the zeolitelayer 150. Thus, there is no chemical reaction between the potassiumfixed to the zeolite layer 150 and the SOx in the exhaust gas. Thisdecreases the consumption of the absorbing agent with this chemicalreaction, and maintains a high NOx absorbing performance of thecatalyst.

The zeolite layer 150 may be formed of various types of zeolite such asMFI type, Y type, X type, mordenite and ferrielite. In this case,zeolite conforming to the composition of the exhaust gas is selected inview of the structure relevancy with the absorption HC specie.

The cation exchange ability of the zeolite is in inverse proportion to aratio SiO₂/AlO₂ of the zeolite, and the heat-resisting performance ofthe zeolite is in proportion to this ratio. Therefore, this ratio isincreased as much as possible in order to improve the heat-resistingperformance according to the present embodiment. Preparing thecomponents of the zeolite in such a way as to decrease the ratioSiO₂/AlO₂ makes it possible to improve the absorbing agent acquisitionperformance of the zeolite. This reduces the dissipated amount of theabsorbing when the catalyst is used at a high temperature for a longperiod of time.

In order to evaluate the durability and the absorbing agent dispassionpreventing performance of the exhaust gas purifying catalyst accordingto the present embodiment in which the zeolite layer 150 was formedbetween the cordierite carrier 110 and the catalyst layer 130, an NOxcatalyst in which the zeolite was provided as an inhibition layerbetween the catalyst layer, to which the absorbing agent including thepotassium was added, and the cordierite carrier was manufactured, andthe potassium content of an unused NOx catalyst was found by an XRFmethod. Moreover, a bench test and a vehicle running test were conductedwith respect to an engine provided with the NOx catalyst. As a result,the potassium content of the NOx catalyst that was used at a hightemperature for a long period of time was found, and the dissipatedamount of the potassium was found by dividing a difference in thepotassium content before and after the use of the NOx catalyst was foundas the potassium by the original potassium content.

FIG. 12 shows the result of an experiment conducted with respect to thecatalyst of the present embodiment having the zeolite layer 150 and theresults of experiments conducted with respect to an original catalyst inwhich the catalyst layer is held on the carrier, the catalyst of thethird embodiment having the silica layer 120 and the catalyst of thefourth embodiment having the titania layer 140.

As shown in FIG. 12, the dissipated amount of the potassium in theoriginal catalyst was dozens of % to 50%, whereas the dissipated amountof the potassium in the catalyst of the present embodiment was ten plusseveral %. This means that the dissipated amount of the potassium as theabsorbing agent from the catalyst can significantly be reduced. Thedissipated amount of the potassium in the catalyst of the third andfourth embodiments was twenty plus several %.

As is the case with the third and fourth embodiments, the catalyst ofthe present embodiment was subjected to a bench test and a vehiclerunning test, and then, an elementary analysis was conducted withrespect to a cut surface of the catalyst by the EPMA method. As aresult, it was found that the potassium added to the catalyst layer 130was prevented from permeating the cordierite carrier 110 even if thecatalyst was used at a high temperature for a long period of time.

It should be understood, however, that there is no intention to limitthe invention to the specific forms disclosed, but on the contrary, theinvention is to cover all modifications.

For example, the honeycomb cordierite carrier is used as a porouscarrier in the above embodiments, but the present invention can also beapplied to an exhaust gas purifying catalyst provided with a carrierformed of material other than the cordierite. The use of a metal carrierprevents the splash of the absorbing agent and prevents thedeterioration of the exhaust gas purifying performance of the catalyst,although the prevention of the permeation of the NOx absorbing agentinto the carrier is nearly out of question. The honeycomb cordieritecarrier is used, the shells thereof are not only quadrangular but alsotriangular and hexagonal.

The silica layer 120, which is comprised mainly of silicon dioxide,constitutes the inhibition layer according to the third embodiment; thetitania layer, which is mainly comprised of titanium dioxide,constitutes the inhibition layer according to the fourth embodiment; andthe zeolite layer 150 constitutes the inhibition layer according to thefifth embodiment. The components of the inhibition layer, however,should not be restricted to the silicon dioxide, the titanium dioxideand the zeolite.

More specifically, the inhibition layer may be composed of a layer witha high acidity by using other acid material than the silicon oxide. Thehabiting layer may also be composed of a layer with a high basicity,which is mainly comprised of basic materials (e.g., alkali metal such asbarium (Ba) and barium oxide (BaO), instead of the titanium oxide.Moreover, the inhibition layer may be composed of a layer with a largespecific surface, which is comprised mainly of a material with a largespecific surface such as zeolite; a layer composed of an elementcompound with a heavy molecular weight, which is comprised mainly of astable basic material with a heavy molecular weight such as a bariumsulfate; and a layer with a small crystal lattice.

In a wide sense, the present invention can form the inhibition layerfrom an acid oxide including an acid substance; a composite oxideincluding an acid substance; and a material that never disturbs areactivity between a nitrogen oxide and said absorbing agent; and amaterial that absorbs a reduced substance. The acid substance mayinclude at least one material selected from transition elements of IV, Vand VI groups and typical elements of IV, V and VI groups.

According to the third through fifth embodiments, one inhibition layer120, 140 or 150 is formed on the external surface of the carrier 110between the carrier 110 and the catalyst layer 130, but the number andposition of inhibition layers should not be restricted to this. Forexample, one inhibition layer may be formed on the external surface ofthe catalyst layer. In the case of a catalyst with a plurality ofcatalyst layers, one or more inhibition layers may be formed at leastone position between the carrier and the catalyst layer, inside thecatalyst layer or on the external surface of the catalyst layer.

What is claimed is:
 1. An exhaust gas purifying catalyst, comprising: aporous carrier; an inhibition layer on said porous carrier, saidinhibition layer including an inhibiting agent; and a catalyst layer onsaid inhibition layer, said catalyst layer including an absorbing agentcomprising potassium as one of alkali metal elements, wherein theinhibiting agent in said inhibition layer inhibits movement of saidabsorbing agent in said catalyst layer into said porous carrier.
 2. Anexhaust gas purifying catalyst according to claim 1, additionallycomprising said inhibiting agent dispersed and mixed in said catalystlayer.
 3. An exhaust gas purifying catalyst according to claim 2,wherein said inhibiting agent includes an acid oxide including at leastone acid substance selected from transition elements of IV, V and VIgroups and elements of IV, V and VI groups; a composite oxide includingsaid at least one acid substance; and at least one material selectedfrom the group composed of a material that never disturbs a reactivitybetween a nitrogen oxide and said absorbing agent and a material thatabsorbs a reduced substance.
 4. An exhaust gas purifying catalystaccording to claim 2, wherein: said inhibiting agent includes zeolite.5. An exhaust gas purifying catalyst according to claim 2, wherein: saidinhibiting agent includes an acid oxide comprised of at least one acidsubstance among silica, titanium and tungsten.
 6. An exhaust gaspurifying catalyst according to claim 2, wherein: said absorbing agentincludes potassium; and said carrier is comprised of a porous carrier.7. An exhaust gas purifying catalyst according to claim 1, wherein alayer of said inhibiting agent is formed on an external surface of saidcatalyst layer.
 8. An exhaust gas purifying catalyst according to claim7, wherein the layer of said inhibiting agent on the external surface ofsaid catalyst includes an acid oxide including at least one acidsubstance selected from transition elements of IV, V and VI groups andelements of IV, V and VI groups; a composite oxide including said atleast one acid substance; and at least one material selected from thegroup composed of a material that never disturbs a reactivity between anitrogen oxide and said absorbing agent and a material that absorbs areduced substance.
 9. An exhaust gas purifying catalyst according toclaim 7, wherein said inhibiting agent in the layer on the externalsurface of said catalyst includes zeolite.
 10. An exhaust gas purifyingcatalyst according to claim 7, wherein said inhibiting agent in thelayer on the external surface of said catalyst includes an acid oxidecomprised of at least one acid substance among silica, titanium andtungsten.
 11. An exhaust gas purifing catalyst according to claim 1,wherein said inhibiting agent includes zeolite.
 12. An exhaust gaspurifying catalyst according to claim 1, wherein said layer of saidinhibiting agent includes an acid oxide comprised of at least one acidsubstance among silica, titanium and tungsten.
 13. An exhaust gaspurifying catalyst according to claim 1, wherein said inhibiting agentincludes an acid oxide including at least one acid substance selectedfrom transition elements of IV, V, and VI groups and elements of IV, Vand VI groups; a composite oxide including said at least one acidsubstance; and a least one material selected from the group composed ofa material that never disturbs a reactivity betweeen a nitrogen oxideand said absorbing agent and a material that absorbs a reducedsubstance.